The growth, the development and the yield of a crop or an ornamental plant depends on the energy that the plant gains through the fixing of CO.sub.2 in carbohydrates during photosynthesis. The primary loci for photosynthesis are the leaves and to a lesser extent the stem tissue, whereas other organs of the plant, such as roots, seeds or tubers, do not make a material contribution to the formation of photoassimilates, but on the contrary are dependent for their growth on the supply from photosynthetically active organs. This means that there is a flow in photosynthetically gained energy from photosynthetically active tissues to photosynthetically inactive parts of a plant.
The photosynthetically active tissues are designated as sources. They are defined as net exporters of the fixed carbon dioxide. The photosynthetically inactive parts of a plant are designated as sinks. They are defined as net importers of the photosynthetically fixed carbon dioxide.
It is believed that the sinks have a strong influence in several ways both in the efficient use of photosynthetic products as well in their distribution within a plant. One example is the habit of the plant. Newly developing organs, such as very young leaves or other areas such as roots and seeds, are fully dependent on the photosynthesis performance of the sources. That means that the development of such organs is dependent on the distribution of the photoassimilates formed from the sources within the plants. The possibility of the formation of young leaves or also the formation of roots can have drastic effects on the habit of a plant, such as for example the size of a plant, the internode separation, the size and shape of a leaf, the appearance of a leaf and the number and shape of the roots. Further, the distribution of photoassimilates would have quite critical meaning for the yield of a plant. Whilst, in the last decades, the harvestable yield of a wheat plants has increased, the total photosynthesis performance of wheat has not changed significantly. This is explained by the sink to source relationship being changed in such a way that the sinks which are important for the yield, such as seeds, take up essentially more photoassimilates than other parts of the plant which are unimportant as far as yield is concerned, such as the stem. In this case, through a shortening of the stem, a much more valid sink to source relationship in wheat could be achieved. This underlines the importance of the distribution of photoassimilates in higher plants formed in the primary sources in relation to both the habit and also the yield of plants.
It is not known through which biochemical mechanism the relationship of sink and source is regulated.
New biotechnological processes for the genetic change of dicotyledonous and monocotyledonous plants are known (Gasser and Fraley, 1989, Science 244, 1293-1299).
In most plants, photoassimilates are distributed within a plant in the form of sugars and preferentially in the form of sucrose. The distribution of sucrose between source and sink tissues occurs by transport of sucrose via the phloem. One of the important determinants for the strength of a sink could be the unloading of the phloem in the sink. In order to achieve a strong unloading of sucrose from the phloem into the sink, the sucrose should be transformed as soon as possible after leaving the phloem into a different chemical component that no longer has a chemical relationship to sucrose.
Changes of the plant habit mean important improvements in known plants. For example, this can lead to a shortening of the stem to give varieties which have greater wind resistance. A preferable distribution of the photoassimilates in harvestable organs such as seeds, eg of barley, wheat, soya beans or maize; leaves, for example tobacco; stems, for example sugar cane; tubers, for example potatoes; beets, for example animal feed beets and sugar beet; and fruit, for example tomatoes, should lead to a higher yield of a plant. These changes apply also to ornamental and garden plants which lead to plants with a completely new habit.